The present invention relates to a method for removal of ammonia from a gas mixture formed by catalytic production of ammonia. The gas mixture consists mainly of unconverted synthesis gas, some ammonia, inert gases and possibly also water. Ammonia and possibly also water are removed from the gas mixture by absorption with an organic absorption agent having two or more OH-groups, preferably a glycol.
Ammonia is produced by catalytic reaction between nitrogen and hydrogen. A commonly used synthesis pressure is about 200 bar. The conversion is not complete and unconverted synthesis gas has to be recirculated to the reactor. The ammonia formed must accordingly be separated from the gas mixture. Some ammonia can be recirculated. The problem is to remove as much of the ammonia as possible in an economical way, and it is especially important to have effective ammonia removal when the synthesis is carried out at relatively low pressure.
Another problem is the water which gets into the gas mixture from earlier stages in the process, especially the methanation stage. The water in the synthesis gas will deactivate the catalyst. Accordingly, it is desirable to remove as much as possible the water which might be present, before the gas reaches the synthesis reactor.
Ammonia can be removed from the gas mixture in several ways. The most used one, especially in high pressure units, is to remove the ammonia from the synthesis gas by condensation with cooling water. However, only part of the ammonia will be removed in each recirculating step. If lower synthesis pressure is used, a cooling unit requiring a high input of energy to its compressor is used, and the costs of apparatus utilized, for instance heat exchangers, will be high.
Another method which has been proposed is to wash out the ammonia with water, for instance as described in GB 2.067.175 A. The ammonia can be separated from the water by distillation. The main disadvantage of this method is that the gas mixture is moistened by the water. The result of this is that the gas mixture has to be dried subsequent to the ammonia removal to avoid deactivation of the catalyst. A molecular sieve is used for removing water from the gas mixture.
Another disadvantage of this type of process is that the heat of absorption and heat of desorption for ammonia in water is high. The result of this is that a large amount of energy has to be supplied for separation of the ammonia from the water.
Further it is known from DE 19 24 892 to remove ammonia from partly converted synthesis gas by absorption in a solvent, for instance ethylene glycol. The solvent can be regenerated by heating with steam. Desorption is carried out in a column having a lower pressure.
The disadvantage of this method is that desorption is carried out under such low pressure that the cooling water can not be used in the subsequent condensation of ammonia. Ammonia must therefore be compressed or re-absorbed in order to get liquid ammonia, and this is expensive.
Another problem with this method is that the desorption temperature when using glycol has an upper limit of 170.degree. C. because of the danger of decomposition. If high is applied pressure during the desorption such that it is possible to use cooling water for condensation of ammonia, the solvent will not be completely regenerated and accordingly part of the ammonia will be recirculated back to the synthesis.